TITLE: Polyamide membrane and process for its production
European Patent EP0090483
 B2

ABSTRACT:
Abstract of EP0090483
Surface modified, skinless, hydrophilic, microporous, polyamide
membranes are formed by preparing a casting solution comprised of (A) a
casting resin system comprised of (a) an alcohol-insoluble polyamide
resin, and (b) a cationic, water-soluble, quaternary ammonium,
thermosetting, membrane surface modifying polymer, and (B) a solvent
system in which the casting resin system is soluble; inducing
nucleation of the casting solution by controlled addition of a
non-solvent for the casting resin system under controlled conditions to
obtain a visible precipitate of casting resin system particles, thereby
forming a casting composition; spreading the casting composition on a
substrate to form a thin film; contacting and diluting the film of the
casting composition with a liquid non-solvent system for the casting
resin system, thereby precipitating the casting resin system from the
casting composition in the form of a thin, skinless, hydrophilic,
surface modified, microporous, polyamide membrane; and washing and
drying the membrane. The membranes of this invention are characterized
by having fine pore ratings, the surfaces thereof being substantially
completely covered by the modifying polymer, a positive zeta potential
in alkaline media, and for those with moderate or low levels of surface
modifying polymer present, a time to reach an effluent resistivity of
14 megaohms/cm under the Resistivity Test of 10 minutes or less. They
have greatly enhanced filtration efficieny over a broad pH range with a
variety of contaminants, including very fine negatively charged
particles, bacteria and endotoxins.

INVENTORS:
Gsell, Thomas Charles (9 Grace Lane, Levittown, New York, 11756, US)
Joffee, Irving Brian (19 Clearview Street, Huntington, New York, 11743,
US)
Degen, Peter John (24 Glades Way, Huntington, New York, 11743, US)

APPLICATION NUMBER: EP19830300518
PUBLICATION DATE: 08/21/1991
FILING DATE: 02/02/1983

ASSIGNEE: PALL CORPORATION (30 Sea Cliff Avenue, Glen Cove, New York, 11542, US)
INTERNATIONAL CLASSES: C08L77/00; B01D39/16; B01D61/22; B01D67/00; B01D71/56; C08J9/28;
(IPC1-7): B01D69/00
EUROPEAN CLASSES: B01D71/56; B01D67/00K14; B01D67/00K14B

DOMESTIC PATENT REFERENCES:
EP0005536 Process for preparing polyamide membrane filter media and
 products thereof.
EP0050789 Liquophilic polyamide membrane filter media and process of
 preparation thereof.
EP0050864 A hydrophilic cationic charge modified microporous membrane,
 a process for producing it and its use.

FOREIGN REFERENCES:
2926154 Cationic thermosetting polyamide-epichlorohydrin resins and
  process of making same
3224986 Cationic epichlorohydrin modified polyamide reacted with
  water-soluble polymers
4340479 Process for preparing hydrophilic polyamide membrane filter
  media and product
Attorney, Agent or Firm:
Corin, Christopher John (Mathisen Macara & Co. The Coach House 6-8
Swakeleys Road, Ickenham, Uxbridge, UB10 8BZ, GB)

CLAIMS:
Claims for the following Contracting States: BE NL SE
1. A surface-modified, skinless, hydrophilic, microporous,
alcohol-insoluble polyamide membrane derived from an alcohol-insoluble
hydrophobic, polyamide resin having a ratio (CH[2]:NHCO) of methylene
CH[2] to amide NHCO groups within the range of from 5:1 to 7:1, said
membrane having (1) the surface properties thereof substantially
controlled by cationic quaternary ammonium groups of a cationic,
quaternary ammonium, thermoset, surface-modifying polymer, thereby
providing a positive zeta potential in alkaline media, and (2) a time
to reach an effluent resistivity of 14 megaohms/cm under the
Resistivity Test of 10 minutes or less.
2. A membrane according to claim 1 having through pores extending from
surface to surface that are substantially uniform in shape and size.
3. A membrane according to claim 1 having through pores extending from
surface to surface that are tapered, being wider at one surface and
narrowing as they proceed toward the opposite surface of the membrane.
4. A membrane according to any one of claims 1 to 3 wherein said
polyamide resin is polyhexamethylene adipamide, poly-e-caprolactam or
polyhexamethylene sebacamide.
5. A membrane according to any one of claims 1 to 4 comprising 80 to
99.9% of the alcohol-insoluble polyamide resin and from 20 to 0.1% by
weight of the surface-modifying polymer.
6. An assembly comprising membranes according to any one of claims 1 to
5, two or more of said membranes being secured to each other and
forming a multiple layer membrane filter sheet.
7. A process for preparing a surface-modified, skinless, hydrophilic,
microporous, alcohol-insoluble polyamide membrane according to claim 1,
the process comprising the steps of: (1) preparing a casting solution
including (A) a casting resin system comprised of (a) an
alcohol-insoluble polyamide resin having a ratio (CH[2]: NHCO) of
methylene CH[2] to amide NHCO groups within the range of from 5:1 to
7:1, and (b) a cationic, water-soluble, quaternary ammonium,
thermosetting, membrane-surface-modifying polymer and (B) a solvent
system in which said casting resin system is soluble; (2) inducing
nucleation of said casting solution by controlled addition of a
non-solvent for said casting resin system under controlled conditions
of concentration, temperature, addition rate and degree of agitation to
obtain a visible precipitate of casting resin system particles, thereby
forming a casting composition; (3) spreading said casting composition
on a substrate to form a film thereof on the substrate; (4) contacting
and diluting the film of said casting composition with a liquid
non-solvent system for said casting resin system comprised of a mixture
of solvent and non-solvent liquids, thereby precipitating on to the
substrate said casting resin system from said casting composition in
the form of said thin, skinless, hydrophilic, surface modified,
microporous, polyamide membrane; (5) washing said membrane to remove
the solvent: and (6) drying said membrane.
8. A process according to claim 7 wherein the precipitated casting
resin system particles are redissolved or are filtered out before
spreading said casting composition on said substrate.
9. A process according to claim 7 or 8 wherein said polyamide resin is
polyhexamethylene adipamide, poly-e-caprolactam, or polyhexamethylene
sebacamide.
10. A process according to any one of claims 7 to 9 wherein said
solvent system for said casting resin system comprises formic acid, and
said non-solvent added to induce nucleation is water.
11. A process according to any one of claims 7 to 10 wherein said
solvent system for said casting resin system comprises formic acid and
water.
12. A process according to any of claims 7 to 11 wherein said
membrane-surfacing-modifying polymer is a polyamine epichlorohydrin
polymer, or a polyamido/polyaminoepichlorohydrin polymer.
13. A process according to any one of claims 7 to 12 wherein said
casting composition is continuously spread onto said substrate, said
film of said casting composition is continuously immersed in a bath of
said liquid non-solvent system, and the bath is maintained at a
substantially constant composition with respect to non-solvent and
solvent by the addition of non-solvent to the bath in a quantity
sufficient to compensate for solvent diffusion into the bath from said
film of said casting composition.
14. A process according to claim 13 wherein the substrate is a porous
web having an open structure which is wetted and impregnated by the
casting composition, thereby forming a membrane film having the porous
web incorporated as a part thereof.
15. A process according to any one of claims 7 to 14 wherein the
substrate is stripped from the membrane when the process has been
completed.

DESCRIPTION:
The present invention relates to microporous membranes, and their
preparation. Microporous membranes have been recognised for some time
as useful for filtering fine particles from gas and liquid media.
United States Patent Specification no. 4,340,479, discloses a process
for manufacturing microporous polyamide membranes with certain
desirable filtration characteristics. Membranes prepared by this
process are hydrophilic, have narrow pore size distributions and pore
ratings as fine as about 0.04 micrometer. For many filtering
requirements those membranes perform very effectively. For certain fine
particulates, e.g., substantially below 0.1 micrometer in diameter,
they are not effective. The reasons for this are related to the
mechanisms by which filters work.
Similar remarks and those which follow also apply to membranes as
described in EP A-O 050 864. These membranes are cationic
charge-modified and comprise a hydrophilic organic polymeric
microporous membrane and a charge-modifying agent bonded to
substantially all of the wetted surfaces of the membrane, the primary
charge modifying agent being a water soluble organic polymer having a
molecular weight greater than about 1000, wherein each monomer thereof
has at least one epoxide group capable of bonding to the surface of the
membrane and at least one tertiary amine or quaternary ammonium group.
Superficially, EP-A 0 050 864 describes a membrane in which the
charge-modifying agent bonds throughout the thickness of the membrane.
The process for achieving this alleged homogeneity simply requires that
a prepared hydrophilic polymeric microporous membrane should be
immersed in an aqueous solution of a water soluble organic polymer
having a molecular weight greater than about 1000. Tests on membranes
in accordance with preferred embodiments of this prior specification
show, however, that the modifying agent does not, in fact, become
distributed homogeneously throughout the thickness of the membrane. As
a result the rate at which high resistivity is achieved is appreciably
lower than is desirable from the point of view of high output with a
reasonable amount of equipment. Corresponding disadvantages will be
encountered when other fluids are filtered.
The function of a filter is the removal of suspended particulate
material and the passage of the clarified fluid medium. A filter
membrane can achieve fluid clarification by different mechanisms.
Particulate material can be removed through mechanical sieving wherein
all particles larger than the pore diameter of the filter membrane are
removed from the fluid. With this mechanism, filtration efficiency is
controlled by the relative size of the contaminant and filter pore
diameter and the efficient removal of very small particles, e.g., less
than 0.1 micrometer, in diameter, therefore requires filter membranes
with very small pore sizes.
Such fine pore filter membranes tend to have the undesirable
characteristics of high pressure drop across the filter membrane,
reduced dirt capacity and shortened filter life.
A filter may also remove suspended particulate material by adsorption
onto the filter membrane surfaces. Removal of particulate material by
this mechanism is controlled by the surface characteristics of (1) the
suspended particulate material and (2) the filter membrane. Most
suspended solids which are commonly subjected to removal by filtration
are negatively charged in aqueous systems. This feature has long been
recognized in water treatment processes where cationic flocculating
agents, oppositely charged to the suspended matter, are employed to
improve settling efficiences during water clarification.
Colloid stability theory can be used to predict the interactions of
electrostatically charged particles and surfaces. If the charges of
suspended particle and the filter membrane surface are of like sign and
with zeta 35 potentials of greater than about 20 mV, mutual repulsive
forces will be sufficiently strong to prevent capture by adsorption. If
the zeta potentials of the suspended particle and the filter membrane
surface are small, or more desirably of opposite sign, particles will
tend to adhere to the filter membrane surfaces with high capture
efficiencies. Most suspensions of particles encountered in industrial
practice have a negative zeta potential. This microporous filter
membranes characterised by positive zeta potentials are capable in a
large number of industrial applications of removing particles much
smaller than the pore diameters of the membrane through the mechanism
of electrostatic capture.
The desirable hydrophilic properties of the polyamide membranes of U.S.
patent specification 4,340,479 are believed to result in part from the
high concentration on the exposed membrane surfaces of amine and
carboxylic acid end groups of the polyamide. The positioning of these
groups is also believed to provide these membranes with their unusual
zeta potential versus pH profile. That profile, positive at pHs below
6.5, becomes negative in alkaline media. Accordingly, these membranes
have limited ability to filter very fine negatively charges
particulates in alkaline media.
By modifying the surface characteristics of the hydrophilic membranes
disclosed in U.S. patent specification 4,340,479 to, for instance,
provide a strongly positive zeta potential over the alkaline range, the
spectrum of uses for these materials in filtration is substantially
expanded.
The present invention is directed, then, to a surface-modified,
skinless, hydrophilic, microporous, alcohol-insoluble polyamide
membrane as recited in claim 1.
The invention also comprises an assembly comprising membranes according
to the immediately preceding paragraph, two or more of said membranes
being secured to each other and forming a multiple layer membrane
filter sheet.
According to the present invention there is also provided a process for
preparing a polyamide membrane according to the last but one preceding
paragraph comprising the steps of:
  (1) preparing a casting solution including (A) a casting resin system
comprised of (a) an alcohol-insoluble polyamide resin having a ratio
(CH[2]:NHCO) of methylene CH[2] to amide NHCO groups within the range
of from 5:1 to 7:1, and (b) a cationic, water-soluble, quaternary
ammonium, thermosetting, membrane-surface-modifying polymer and (B) a
solvent system in which said casting resin system is soluble;
  (2) inducing nucleation of said casting solution by controlled
addition of a non-solvent for said casting resin system under
controlled conditions of concentration, temperature, addition rate and
degree of agitation to obtain a visible precipitate of casting resin
system particles, thereby forming a casting composition;
  (3) spreading said casting composition on a substrate to form a film
thereof on the substrate;
  (4) contacting and diluting the film of said casting composition with
a liquid non-solvent system for said casting resin system comprised of
a mixture of solvent and non-solvent liquids, thereby precipitating on
to the substrate said casting resin system from said casting
composition in the form of said thin, skinless, hydrophilic,
surface-modified, microporous, polyamide membrane;
  (5) washing said membrane to remove the solvent; and
  (6) drying said membrane.
The surface modified, alcohol-insoluble polyamide membranes in
accordance with this invention have the unusual property of being
hydrophilic, i.e., they are readily wetted by water, have pore sizes
(also referred to as pore ratings or pore diameters) of from 0.04 to 10
(preferably 0.1 to 5) micrometers or more, modified zeta potentials,
i.e., strongly positive zeta potentials in alkaline media, filtration
efficiencies ranging from molecular dimensions (pyrogens) to
particulates larger than the pore diameters and, accordingly, are
highly desirable as filter media, particularly for producing
bacterially sterile filtrates, as well as for filtration of high purity
water in microelectronics manufacture due to their ability to deliver
ultrapure filtrate free from microparticulates and ionic contaminants.
The membrane surface modifying polymers or resins useful in preparing
the membranes in accordance with this invention are the cationic,
water-soluble, quaternary ammonium, thermosetting polymers. Preferred
polymers within this class are the epoxy-functional
polyamide/polyamidoepichlorohydrin resins. The epoxy-functional
polyamine epichlorohydrin resins are particularly preferred.
The sole figure is a graph of percent added surface modifying polymer
versus time required to obtain 14 megaohms/cm effluent water from a
surface modified membrane in accordance with this invention.
The polyamide membranes of U.S. Patent Specification 4,340,479 are
prepared from alcohol-insoluble polyamide resins having a methylene to
amide ratio in the range of 5:1 to 7:1. as are the surface, modified
membranes in accordance with this invention. Membranes of this group
include copolymers of hexa-methylene diamine and adipic acid (nylon
66), copolymers of hexamethylene diamine and sebacid acid (nylon 610)
homopolymers of poly-e-caprolactam (nylon 6) and copolymers of
hexamethylene diamine and azelaic acid (nylon 69). Nylon 66 is
preferred.
In the process for manufacturing the membranes of the U.S. Patent
Specification 4,340,479, the polyamide resin is dissolved in a solvent,
such as formic acid, and a non-solvent, such as water, is added under
controlled conditions of agitation to achieve nucleation of the
solution.
In inducing nucleation of the polyamide solution a visible precipitate
is formed. This precipitate may partially or completely redissolve.
Preferably, any visible particles which do not redissolve should be
filtered out of the system, e.g. with a 10 micrometer filter, prior to
casting the nucleated solution or casting composition.
The nucleated solution or casting composition is then cast onto a
substrate, e.g., a porous polyester sheet of web or a non-porous
polyester sheet, in the form of a film and this film solution is then
contacted with and diluted by a liquid non-solvent system which is a
mixture of a solvent and a non-solvent for the polyamide resin. A
preferred non-solvent liquid system for both the subject invention and
that of the U.S. Patent Specification 4,340,479 is a solution of water
and formic acid. For this invention, the formic acid is preferably
present in an amount of from 35% to 60% by weight. The polyamide resin
thereupon precipitates from the solution, forming a hydrophilic
membrane sheet on the substrate which can be washed to remove the
solvent. The membrane can then be stripped from the substrate and dried
or, if the substrate is porous, it can be incorporated in the membrane
to serve as a permanent support, in which event it is dried with the
membrane. If the substrate is to be incorporated into the membrane, it
should be porous and capable of being wetted and impregnated by the
casting composition, e.g., a porous, fibrous, polyester sheet with an
open structure. By appropriate control of process variables, membranes
with through pores of uniform size and shape can be obtained.
Conversely, if desired, tapered through pores, wider at one surface of
the sheet and narrowing as they proceed toward the opposite surface of
the sheet, can be obtained.
The same general procedure described above is followed in manufacturing
the surface modified membranes in accordance with this invention except
that the membrane surface modifying polymers used in the subject
invention are combined with the polyamide resin and the resulting
combined modifying polymer/polyamide casting solution, after nucleation
to form the casting composition, is cocast resulting in unique
membranes with novel filtration properties extending the range of uses
for microporous polyamide membranes.
The novel properties of the filter membranes prepared by the process of
U.S. Patent Specification 4,340,479 are believed to result in part from
the high concentration on the membrane surfaces of amine and carboxylic
acid end groups of the polyamide. These amine and carboxylic acid
functions on the membrane surfaces result in unexpected membrane
properties, such as their unusual zeta potential versus pH profile and
their hydrophilic character, that is being readily wetted by water,
typically being wetted through in 3 seconds or less, preferably 1
second or less, when immersed in water.
As previously stated, it has now been discovered that the surface
modified membranes in accordance with this invention having unexpected
and novel filtration properties can be prepared using the general
procedure disclosed in U.S. Patent Specification 4,340,479 but with the
addition of low levels of selected membrane surface modifying polymers
to the polyamide membrane casting solutions. Thus, surface modified,
hydrophilic, microporous polyamide membranes with a strongly positive
zeta potential in alkaline media, having low levels of extractable
matter, and having the ability to deliver ultrapure water, free from
microparticulate and ionic contaminants, quickly after the onset of
filtration as required in microelectronics manufacture, are readily and
economically prepared by the cocasting process in accordance with the
present invention.
Addition of as little as one weight percent, based on the polyamide
resin, of the membrane-surface-modifying polymer to the membrane
casting solution has been found to produce microporous hydrophilic
membranes whose surface properties are substantially controlled by the
modifying polymer. It is the ability of relatively small amounts of the
membrane surface modifying polymer to control the surface properties of
membranes in accordance with the present invention which provides the
desirable characteristics of the subject membranes. Accordingly, the
filtration characteristics and the physiochemical surface behaviour of
these membranes are controlled by a surprisingly low proportion of the
modifying polymer.
The (1) surface modified, polyamide membranes prepared using the
cocasting process share certain characteristics with both (2) the base
polyamide membranes prepared using the same general casting process but
without the modifying polymer present in the casting solution and (3)
base polyamide membranes prepared in the same mamner as the membranes
of (2) above but which have subsequently been coated with a cationic
polymer or resin by contacting the finished base polyamide membrane
with a solution of the cationic polymer or resin.
All three types are skinless, hydrophilic (as hereinbefore defined and
in U.S. Patent Specification 4,340,479) and microporous. Each has
desirable filtering characteristics for fine particulates derived in
part from their fine pore ratings and narrow pore size distributions.
Unlike the base polyamide membranes of (2) above, however, the surface
modified polyamide membranes of (1) above and the coated polyamide
membranes of (3) above exhibit a positive zeta potential in alkaline
media which broadens the spectrum of uses of the membranes of (1) and
(3).
Surface modified polyamide membranes in accordance with this invention
and base polyamide membranes (2) also possess the ability to deliver
ultrapure water, free from microparticulate and ionic contaminants,
quickly after the onset of filtration. Conversely, the coated polyamide
membranes (3) do not have this ability to as great an extent as either
(1) or (2). This ability, as described in detail hereinafter, is highly
desirable in microelectronics manufacturing applications. Since only
the surface modified polyamide membranes (1) in accordance with this
invention have the unique combination of a positive zeta potential in
alkaline media and the ability to deliver ultrapure water, free from
microparticulate and ionic contaminants, quickly upon the onset of
filtration, the desirability of this class of membrane is manifest. The
highly desirable properties of membranes in accordance with the
invention are believed to result from the unique method of preparation
in which the modifying polymer becomes an integral part of the overall
structure of the membrane. The ability to prepare these membranes in a
clean, straightforward, efficient and economic manner heightens their
desirability.
The membrane-surface-modifying polymers or resins (sometimes herein
referred to as "modifying polymer(s)") useful in processes in
accordance with this invention are the cationic, water-soluble,
quaternary ammomium, thermosetting polymers. The preferred modifying
polymers are those polymers which undergo cross-linking reactions
through reaction of epoxide groups. Epoxide functional cationic
polymers or resins generally produce charge-modified surfaces which,
upon proper conversion to the cross-linked state by the application of
heat, are found to be mechanically strong and chemically resistant to a
wide range of aggressive chemical environments.
Epoxide functional or epoxide-based cationic thermosetting polymers are
also preferred due to favourable interactions believed to occur between
the amine and carboxylic acid end groups of the polyamide. Amine
functions and carboxylic acid functions are known to co-react
efficiently with epoxide functional polymers. It is believed that the
amine and carboxylic acid groups of the polyamide resin react with the
epoxide groups of the modifying polymer. While it is believed that the
reaction of these groups occurs throughout the membrane structure, the
nature of the membrane forming process is believed to cause
preferential orientation of the modifying polymer towards the surfaces
of the formed membrane. By this is meant that as a result of the
cocasting process in accordance with the invention, the modifying
polymer determines the surface characteristics of the membrane.
Further, the reaction of the groups is believed to result in intimate
bonding of the modifying polymer and the polyamide resin forming an
integral structure thereby reducing the amount of extractable matter,
increased homogeneity of the surfaces and increased general stability
of the membranes.
Another desired characteristic of the modifying polymers relates to the
nature of their cationic charge. Since their cationicity stems from the
presence of quaternary ammonium groups, they maintain their positive
charge in acid, neutral and alkaline conditions of pH.
Surprisingly, during the membrane formation process, the low added
levels of modifying polymers appear to be preferentially orientated in
such a manner as to result in membranes whose surface characteristics
are substantially controlled by the modifying polymer. This result is
believed to reflect both the membrane forming process and the
hydrophilic nature of the modifying polymers. The combination of their
hydrophilicity, their apparent strong interaction with the polyamide
end groups and the cocasting process are believed responsible for the
apparent preferential orientation of the modifying polymers towards the
membrane surface.
Also, surprisingly, while the modifying polymers are highly soluble in
water, they are not leached out of the casting composition into the
non-solvent liquid, which is used to precipitate the casting resin
system. Apparently, the strong inter-action of the modifying polymer
with the polyamide end groups coupled with the preferential orientation
of the modifying polymer towards the membrane surfaces (both pore and
external), perhaps under the influence of the non-solvent liquid,
combine to provide a membrane whose surface properties are
substantially controlled by the cationic, quaternary ammonium groups of
the modifying polymer. The unexpected result is highly desirable.
The epoxy-functional or epoxy-based resins preferred fall into two
classes: polyamido/polyamino epichlorohydrin resins and
polyamide-epichlorohydrin resins. The former are reaction products of
epichlorohydrin with polyamides containing primary, secondary and
tertiary amines in the backbone. Representative materials of this class
are described in United States Patent Specifications 2,926,154,
3,332,901, 3,224,986, and 3,855,158.
Preferred commercially available water-soluble, non-colloidal cationic
thermosetting polymers of the polyamido/polyamino-epichlorohydrin
class, are Kymene 557 (Registered Trade Mark) and the Polycup
(Registered Trade Mark) series of resins manufactured by Hercules
Incorporated. It is believed that these resins are prepared by reacting
epichlorohydrin with low molecular weight polyamides which contain
amino groups in the polymer backbone. The reaction products have been
described as containing quaternary ammonium groups present in the form
of azetitinium ions which are four-membered ring structures. Kymene 557
and the members of the Polycup series have been described as being
chemically and structurally similar but differing in their molecular
weight.
Polyamine-epichlorohydrin resins are condensation products of
polyamines such as polyalkylene polyamines or their precursors with
epichlorohydrin. They differ from the polyamido/polyamino
epichlorohydrin resins in that the polyamine-epichlorohydrins contain
no amide linkages as part of the backbone of the polymer. Commercially
available compositions of this type are disclosed in (1) U.S. Patent
Specification 3,885,158, which discloses polymers prepared by reacting
epichlorohydrin with the condensation product of polyalkylene
polyamines and ethylene dichloride and in (2) U.S. Patent Specification
3,700,623 which discloses polymers made by reacting epichlorohydrin
with polydiallymethylamine. Compositions of the first type are
exemplified by Santo-res 31 (Registered Trade Mark) (Monsanto, Inc.)
and compositions of the second type are exemplified by Resin R4308
(Hercules Inc.):
Especially preferred are the epoxide functional water soluble cationic
polymers which fall in the class of polyamine-epichlorohydrin resins
and which bear quaternary groups in the cured state. The fact that
quaternary groups remain in the resin in the cross-linked state is
important, since it affects the pH range over which the membrane can
maintain a positive zeta potential. A quaternary ammonium group is
inherently cationic; hence its positive charge is independent of its pH
environment. Resin R4308 and Santo-res 31 each bear quaternary ammonium
groups in the cured state and bear a positive charge at alkaline pH.
Many of the modifying polymers require activation. For the purpose of
providing extended shelf life or storage stability to these resins, the
epoxide groups are chemically-inactivated to prevent premature
crosslinking of these polymers. Thus, prior to the use of these
polymers the polymers are activated into the reactive, thermo-setting
state by regeneration of the epoxide groups. Typically, activation
entails adding sufficient aqueous caustic to a solution of the inactive
polymer to chemically convert the inactive chlorohydrin form to the
cross-linking epoxide form. The parts by weight of aqueous caustic per
weight polymer vary with the product and are specified by the
manufacturer. Complete activation is generally achieved in about thirty
minutes.
The preparation of membranes in accordance with this invention is
carried out under controlled conditions including addition of the
non-solvent, e.g., water, to a solution of the polyamide and the
modifying polymer, control of the concentration of the constituents,
control of the temperature and control of the agitation of the system
to induce the proper level of nucleation.
The detailed discussion in U.S. Patent Specification No. 4,340,479
concerning the relationship of the parameters set out above is
generally applicable herein and will not be repeated.
The manner and rate of addition of the non-solvent to induce nucleation
is interrelated with other process variables, such as intensity of
mixing, temperature and the concentration of the various components of
the casting solution. The term "casting solution" is used here to mean
the solution made up of (A) the casting resin system and (B) the
solvent system. Addition of the non-solvent is conveniently carried out
through an orifice at a rate sufficient to produce a visible
precipitate which, preferably, at least in part subsequently
redissolves. Maintaining all other parameters constant, casting
compositions with quite different characteristics in terms of pore
sizes of the resulting membranes will be obtained by varying the
diameter of the orifice. The required degree of nucleation resulting
from non-solvent addition rate and orifice configuration is therefore
best established by trial and error for each given system.
The controlled addition of non-solvent is discussed in detail in U.S.
Patent Specification No. 4,340,479. Prior to addition of the
non-solvent to induce nucleation, the casting solution is prepared
comprised of (A) a casting resin system comprised of (a) an
alcohol-insoluble polyamide resin as hereinbefore described and (b) a
modifying polymer or resin and (B) a solvent system. The solvent system
may simply be a solvent for the casting resin system, e.g., formic
acid. Alternatively, the solvent system contains an amount of a
non-solvent, e.g., water. The amount of non-solvent present in the
casting solution is always less than the amount necessary to affect the
stability of the solution.
Prior to casting, nucleation of the casting solution is initiated by
controlled agitation and the controlled addition of non-solvent liquid.
The amount and rate of addition of non-solvent is controlled along with
the intensity of mixing of agitation. The advantage of including
non-solvent as part of the solvent system in making up the casting
solution is that better control of the addition of non-solvent can be
maintained during the inducement of nucleation because smaller amounts
of non-solvent are needed due to the non-solvent already present in the
casting solution. As a result, better control of the addition rate can
be maintained and a more uniform product of any desired pore size can
be obtained.
The casting resin system includes (a) an alcohol-insoluble polyamide
resin having a methylene to amide ratio of from 5:1 to 7:1 and (b) a
surface modifying polymer or resin. All parts and percentages are by
weight unless otherwise stated.
The proportion of modifying polymer to polyamide resin in the casting
solution formed as the first step in the process, based on the
polaymide resin, can vary from as much as 20 weight percent to as
little as 0.1 weight percent, that is 20 parts of modifying polymer to
100 parts of polyamide resin ranging to 0.1 part of modifying polymer
to 100 parts polyamide resin. The generally preferred range of added
modifying polymer is from 1 weight percent to 5 weight percent.
Addition levels of 1 to 2.0 weight percent have been found particularly
desirable. It is believed that this moderate level of modifying polymer
produces substantially complete membrane surface modification resulting
in a membrane whose surface characteristics are substantially
controlled by the cationic, quaternary, ammomium groups of the
modifying polymer. Thus, for the purpose of membrane efficiency and
production economy, the addition of 1 to 2.0 weight percent of the
modifying polymer, based on the polyamide resin, is preferred with the
polyamide resin present in the casting solution in an amount of from
10% to 18% and the surface modifying polymer present in an amount of
from 0.1% to 0.9%, (based on all components present in the solution).
The amount of solvent present in the casting solution formed as the
first step in the process will vary dependent upon the polyamide resin
and the modifying polymer used. In general, the amount of solvent
present will range from 60 to 80 weight percent (based on all
components present in the solution).
It should be understood that the casting solution comprises both (1)
the casting resin system, i.e., the polyamide resin and the modifying
polymer or resin and (2) the solvent system, i.e., a solvent for the
polyamide resin/modifying polymer casting resin system (such as formic
acid) and, if desired, a minor amount of non-solvent for the casting
resin system (such as water).
The amount of non-solvent present in the casting solution will in all
cases be less than the amount in the liquid non-solvent system
(membrane forming bath) used to precipitate the casting resin system
from the casting composition, the casting composition being the
composition formed from the initially prepared casting solution by
inducing nucleation in that solution and, preferably, removing visible
particles from the resulting composition. Generally, when the
non-solvent is water, it will be present in the casting solution in an
amount ranging from zero, preferably at least 5 percent, preferably 10
to 20 percent, and up to 30 percent by weight (again based on all the
components present in the solution).
For a preferred casting solution, a polyamine epichlorohydrin resin or
polymer, preferably Resin R4308, is present in the casting solution in
an amount of from 0.1 to 0.9 percent, polyhexamethylene adipamide is
present in an amount of from 10 to 18 percent, formic acid is present
in an amount of from 65 to 75 percent, and water is present in an
amount of 10 to 20 percent, all parts by weight and based on the total
composition of the casting solution.
The temperature of the casting composition is not critical so long as
it is maintained at a constant value. Generally, however, a decrease in
casting composition temperature produces a higher degree of nucleation.
The intensity of mixing in a given system is a function of a large
number of interrelated variables. For any given system, the mixing
intensity can be expressed in terms of the rotation rate of the
agitator. Such equipment has many forms and designs commonly used in
the mixing art and is difficult to quantify. Thus, trial and error
experimentation involving customary variables is necessary to establish
the operable range of mixing intensities suitable for a particular
system. Typically, using a 6.35 cm rotor operating at a throughput of
about 500 to 1500 grams of solution per minute requires mixing speeds
in the range of from 1500 to 4000 RPM to produce membranes with pore
rating in the range of interest.
The liquid non-solvent system used to dilute the film of casting
composition and thereby precipitate the casting resin system, typically
by immersion in a bath of the liquid non-solvent system, can, and
preferably does, contain a substantial amount of a solvent for the
casting resin system, preferably the one present in the casting
solution. That is, the liquid non-solvent system is comprised of a
mixture of a non-solvent for the casting resin system, e.g., water, and
a solvent for the casting resin system, e.g., formic acid. However on a
percentage basis the amount of solvent present in the liquid
non-solvent system will be less than the amount present in the casting
solution. Typically, the liquid non-solvent system will be comprised of
a non-solvent, e.g., water, present in an amount ranging from 65 to 40
weight percent and a solvent for the casting resin system, e.g., formic
acid, present in an amount ranging from 35 to 60 weight percent. The
formic acid present in the casting solution may amount to from 60 to
80% and the water present therein from 0 to 30% by weight. More
normally expressed the formic acid range is from 65 to 75% and the
water range from 10 to 20% by weight. Preferably, the bath of the
liquid non-solvent system is maintained at a substantially constant
composition with respect to non-solvent and solvent by the addition of
non-solvent to the bath, preferably continuously, in a quantity
sufficient to compensate for solvent diffusion into the bath from the
thin film of casting composition.
The solvent comprising at least part of the solvent system used in the
casting solutions can be any solvent for the casting resin system,
i.e., the combination of the polyamide resin and the modifying polymer.
A preferred solvent is formic acid. Other suitable solvents are other
liquid aliphatic acids, such as acetic acid and propionic acid;
phenols, such as phenol; the cresols and their halogenated derivatives;
inorganic acids, such as hydrochloric, sulfuric and phosphoric;
saturated aqueous or alcohol solutions of alcohol-soluble salts, such
as calcium chloride, magnesium chloride and lithium chloride; and
hydroxylic solvents, including halogenated alcohols.
The only criteria in selecting a solvent are that (1) it should form a
solution of the polyamide resin and the modifying polymer, (2) it
should not react chemically with either the polyamide resin or the
surface modifying polymer and (3) it should be capable of ready removal
from the surface modified polyamide membrane. Practical considerations
also are important, of course. For example, inorganic acids are more
hazardous to work with than are others of the named solvents and
corrosion problems must be dealt with. Since formic acid meets the
criteria listed above and is a practical material as well, it is the
solvent of choice. Due to economy and ease of handling, water is the
non-solvent of choice for use in the solvent system when a non-solvent
is used in the solvent system. In like manner, the preferred
non-solvent added to the casting solution to induce nucleation thereof
is water. And, the preferred non-solvent component of the liquid
non-solvent system used to precipitate the casting resin system from
the film of casting composition is also water for the same reasons it
is the non-solvent of choice in the solvent system.
The membrane products are characterized by being hydrophilic, skinless,
microporous and alcohol-insoluble, with narrow pore size distributions;
filtration efficiencies from molecular dimensions (pyrogens) up to
particulates larger than the pore diameters, pore size ratings of from
about 0.04 to 10 micrometers, a preferred range being from 0.1 to 5
micrometers, film thicknesses in the range of from 0.01 to 1.5
millimeters, preferably from 0.025 to 0.8 mm, and by having a positive
zeta potential over a broad pH range of from 3 to 10. The pores may
extend substantially uniformly from surface to surface both in size and
shape or they may be wider at one surface than the other and thus taper
between the surfaces. Additionally, the membranes can be characterized
as having surface properties which are substantially controlled by
cationic quaternary ammonium groups of the cationic, quaternary
ammonium, thermoset, surface modifying polymer. Surprisingly, the low
added levels of quaternary ammonium, surface modifying polymers produce
membranes whose surface characteristics substantially reflect the
presence of the cationic quaternary ammonium groups. Along with their
excellent pore structure and positive zeta potential, these membranes
have very low levels of extractable matter making them especially
desirable in pharmaceutical and electronic filtration applications.
Additionally, these membranes can be conveniently and economically
prepared by a straightforward, continuous process as described
hereinafter.
The surface modified membranes with moderate or low levels of modifying
polymer have rinse up times to deliver effluent water having a
resistivity of 14 megaohms/cm of 10 minutes or less, and preferably
less than 5 minutes, when tested using the Resistivity Test described
hereinafter.
Surface modified membranes, particularly those with moderate or low
levels of surface modifying polymers present, are believed to have
quick rinse up times because of the apparent interaction between the
surface modifying polymer and the polyamide resin end groups. This
interaction and the resulting integral nature of membranes in
accordance with the invention is believed to lead to a reduction in the
extractables available for sloughing off and being carried through the
filter and into the effluent, a phenomenon believed to occur with
coated membranes.
Method of testing the surface modified membranes of the following examples
The properties of the membranes of the following examples were
evaluated by a variety of test methods as described below:
 (a) Zeta potential
The zeta potentials of membranes were calculated from measurements of
the streaming potential generated by flow of a 0.001 weight percent
solution of KCl in distilled water through several layers of the
membrane secured in a filter sheet or membrane holder. Zeta potential
is a measure of the net immobile electric charge on a membrane surface
exposed to a fluid. It is related to the streaming potential generated
when the fluid flows through the membrane by the following formula (J.
T. Davis et al, interfacial Phenomena, Academic Press, New York, 1963):
Zeta Potential (mV)=4πη/D . E[s]λ/P Where η is the viscosity of the
flowing solution, D is its dielectric constant, λ is its conductivity,
E[s] is the streaming potential and P is the pressure drop across the
membranes during the period of flow. In the following examples, the
quantity 4πη/D was constant, having a value 2.052 x 10̅², or when
converted to Kg./m², the constant must be multiplied by the conversion
factor 703.1 so that the zeta potential can be expressed:
  (b) Capacity of membrane for latex adsorption
A 47 mm diameter disc of membrane was placed in a filter holder with a
filtration area of 9.29 cm² and was then challenged with a suspension
of 0.01 weight percent monodisperse latex spheres in a 0.1 weight
percent Triton X-100 in water solution (Triton X-100 is an adduct of
nonyl phenol with about 10 moles of ethylene oxide). The latex
suspension was pumped through the membrane using a Sage Instrument
Model 341 syringe pump at a rate of 2 milliliters per minute. The latex
effluent was detected by 90 degrees scattering of 537 nm as measured by
a Brice-Pheonix BP 2000 light scattering photometer using a flow cell.
The latex solution was pumped through the membrane until the light
scattering of the effluent began to differ from that observed for a
solution of 0.1% by weight Triton X-100 alone, indicating the start of
penetration of latex particles through the membrane. The latex
adsorption capacity (LAC) was calculated by the following relation: LAC
(mg/cm²)=10×V/929.0 where V is the number of milliliters of 0.01 weight
percent latex suspension which could be passed through the membrane
until passage of latex was observed.
 (c) Bacterial titer reduction
Membranes sterilized by the autoclaving were placed in a suitable
stainless steel filter holder and the filter holder with membrane in
place was exposed to steam at 123°C for 30 minutes followed by a 60
minute exhaust period in a Vernitron/Better Built Century 21 Model 222
laboratory sterilizer. They were then challenged with bacteria at four
levels: 10^6, 10^8, 10^1^0, 10^1² bacteria per square metre for a total
challenge of about 10^1² bacteria per square metre of membrane (U.S.P.
Bacterial Titer Reduction Test).
The effluent was collected under aseptic conditions in a sterile glass
vessel. The number of bacteria in the influent and the effluent was
determined by making serial dilutions of these suspensions and plating
them on a 0.22 micrometer analytical membrane. These membranes were
cultured as Muller-Hinton agar at 38°C for 24 hours to grow colonies of
Serratia Marcescens and for 48 hours to grow colonies of Pseudomonas
diminuta.
The colonies growing on the cultured membranes were counted and the
number of colonies observed was assumed equivalent to the number of
bacteria in the solution plated.
As in the case of latex particle adsorption titer reduction, T[R ]. is
defined as the ratio of influent bacterial count to effluent bacterial
count:
  d) Endotoxin titer reduction test method
A 47 mm, diameter disc of test membrane was prewetted with isopropyl
alcohol and placed in a depyrogenated 47 mm, disc holder of filtration
area 0.000929 m², (0.01 ft²), which had been previously depyrogenated
by heating in an oven at 250°C, for 1 hour. 50 ml pyrogen-free water
was passed through the test membrane and the last 3ـ4 ml were collected
in pyrogen-free glassware and saved as a control for the system. The
membrane was then challenged with successive 10 ml aliquots of E. Coli
055:B5 purified endotoxin at a flow rate of 5 ml/min/929 cm² and the
effluent collected and saved as above. The first aliquot was at a
concentration of 1 mg/ml; each successive portion was 10 times the
concentration of the previous one up to a maximum of 100 mg/ml. The
influent and effluent solutions were diluted with pyrogen-free water as
required and analyzed for the presence of endotoxin by the Limulus
Amebocyte Lysate Test (United States Pharmocopeia XX, 1980 page 888).
  (e) Resistivity of effluent water
 Preliminary preparation
Surface modified, microporous, hydrophilic polyamide membranes were
converted to standard cartridge form by conventional procedures to form
cartridges having 0.70 s m² (7.5 sq. ft.) of filtering area. The
cartridges were then flushed with 0.2 molar ammonium hydroxide at
3.44x1.013x10^5 Pa (50 psi) pressure across the cartridge for 6 minutes
in order to convert the surface modifying polymer to the hydroxide
form. The cartridges were then flushed with 1.5 liters of deionized
water to remove residual ammonium hydroxide and then dried for 12 hours
at 79.4°C (175°F). The cartridges were then ready for testing for their
ability to deliver high purity water effluent by the flowing
Resistivity Test Method.
  Resistivity test
Water of near theoretical resistivity was generated by passing tap
water through a Model MA 18090 deionizing bed (Penfield Inc.) and
through two Unibed ion exchange beds (Culligan Inc.) Test membranes in
the form of standard cartridges were mounted in a cartridge housing of
common design and subjected to a flow of approximately 10.8 liters per
square metre membrane area per minute using the water from the
deionizing system. The effluent water from the elements was monitored
for resistivity with a Model 3418 conductivity cell (Yellow Springs
Instrument Company). The conductivity cell was connected to a Model 31
conductivity bridge (Yellow Springs Instrument Company) which allowed
the direct measurement of effluent resistivity as a function of water
flow time. The time in minutes required to reach an effluent
resistivity of 14 megaohms per centimeter, the generally accepted water
quality limit by the electronics industry, was determined.
 General method I for the preparation by continuous casting of the membranes of
  the following Examples 1ـ12
Nylon 66 resin pellets were dissolved in 98.5% formic acid. Sufficient
activated Resin R4308, as a 5 weight percent solution in water, was
added to bring the relative proportion of Resin R4308 to nylon 66 to
the desired value. This homogeneous casting solution comprised of (1)
the casting resin system, i.e., the nylon 66 plus R4308, and (2) the
solvent system, i.e., formic acid plus water, was tested for its
viscosity at 30°C on a Rion Viscotester with a number 1 rotor (Model
VT-04, available from Extech International Corp., Boston, Mass. U.S.A.)
operating at 63.8 r.p.m. The viscosity was found to be about 6 Pa·s
(6000 Centipoise). After viscosity testing, the casting solution was
delivered by metering pump, at flow rates from 250 grams per minute to
about 1500 grams per minute, into an in-line mixer of conventional
design having a 6.35 cm rotor whose mixing intensity was controlled
over a range of speeds. Simultaneously a non-solvent, water, was added,
as indicated for each example, to the mixer by metered injection to
produce the desired ratio of formic acid to water and to induce
nucleation of the casting solution and obtain a visible precipitate.
Upon exiting the mixer, the resulting casting composition was filtered
through a 10 micrometer filter to remove visible resin particles and
was then formed into a thin film on a moving, porous, fibrous,
polyester, non woven 27.30 cm wide, continuous web by a doctor blade
with about 0.0203 cm spacing. Within less than 3 seconds the coated web
was immersed into a membrane forming bath of the liquid non-solvent
system comprised of a mixture of formic acid and water, as specified
for each example, for approximately 1 to 3 minutes. The bath
concentration was maintained constant by the continuous addition of
water to the bath at the required rate to compensate for solvent
diffusion into the bath from the film of the casting composition.
The nylon membrane so formed on the non-woven, porous polyester support
was washed with water for from about 3 to about 6 minutes to remove
residual formic acid. Excess water was removed from the nylon membrane
by passing it between tensioned rubber wiper blades and the membrane
was wound into suitably sized rolls for storage or further processing.
For filtration application or testing of nylon membrane in flat sheet
form, the membrane sheet was mounted in a restraining frame to prevent
shrinkage in any direction and the membrane was oven dried to 143.3°C
for about 5 minutes. The membranes were also converted to filter
cartridges by known methods and subjected to testing or filtration
application in cartridge form. All parts and percentages herein are by
weight unless otherwise specified.
  Example 1
The continuous casting method, described under General Method I above,
was employed to produce a surface modified, microporous, hydrophilic
polyamide membrane. The casting solution was prepared by mixing about
549 parts of formic acid, 92.2 parts of a 5 weight percent activated
Resin R4308 in water and 108.1 parts of nylon 66 resin pellets. The
mixture was mechanically agitated until homogeneous.
The casting solution was pumped into an in-line mixer operating at 3600
RPM at the rate of 1000 grams per minute while water was injected into
the mixer at the rate of 36.4 grams per minute. The resulting casting
composition was then passed through a 10 micrometer filter to remove
visible particles. The casting composition was maintained at a
temperature of 48.5°C and was delivered onto the polyester web by means
of a doctor blade with an approximately 0.020 cm spacing. The web was
passed by the doctor blade at approximately 15.2 metres per minute and
then into a bath containing 50.1 weight percent formic acid and the
balance water. The membrane was then further processed as outlined in
Method 1, that is, it was washed and then dried in a restraining frame
for 5 minutes at 140 degrees C. and was then ready for filter
application or testing.
  Example 2
The method of Example 1 was repeated but with the casting solution made
up of about 374.5 parts by weight of formic acid, 53.0 parts of 5.82%
activated solution of Resin R4308 in water, and 72.5 parts of nylon 66
resin. The casting solution was pumped at a rate of 500 grams per
minute into an in-line mixer operating at 2718 RPM while water was
injected into it at a rate of 24.9 grams per minute. The resulting
casting composition was maintained at 54.0 degrees C. and was cast onto
the polyester web moving at 10.1 metres per minute. The web was then
immersed in a bath containing 55 percent formic acid, the balance
water, and then further processed as described previously until ready
for filter application or testing.
  Example 3
The method of Example 2 was repeated except that the operating speed of
the in-line mixer was 2717 RPM, and that the water was injected into
the mixer at a rate of 17.9 grams per minute. The casting composition
temperature was controlled at 54.7°C. The coated web was passed into a
bath containing 55.4 weight percent formic acid, balance water, at a
rate of 10.1 metres per minute. The membrane was then further processed
as previously described until ready for filter application or testing.
  Example 4
A casting solution composed of about 73.2 weight percent formic acid,
12.3 weight percent water 14.02 weight percent nylon 66 and 0.43 weight
percent activated resin R4308 was prepared by hereinbefore described
procedures. The casting solution was pumped into the in-line mixer,
operating at 3600 RPM, at the rate of 1000 grams per minute. Water was
injected into the mixer at the rate of 32.9 grams per minute and the
resulting casting composition was maintained at a temperature of
47.6°C. The polyester web was passed by the doctor blade with a 0.020
cm spacing at the rate of 16.8 metres per minutes and into the bath
containing 50.1 weight percent formic acid, balance water. The membrane
was then further processed as hereinbefore described until ready for
filter application or testing.
  Example 5
The membrane forming process of Example 4 was repeated except that the
relative weight nylon 66 and Resin R4308 was adjusted to 2 weight
percent Resin R4308 based on the nylon 66, i.e., there were 2 parts of
Resin R4308 for 100 parts of nylon 66. The casting solution was
delivered to the mixer at the rate of 1000 grams per minute and the
resulting casting composition temperature was maintained at 47.5°C. The
web, moving at 16.8 metres per minute, was immersed in a bath
containing 49.6 weight percent formic acid, balance water. The membrane
was then further processed as hereinbefore described until ready for
testing or filter application.
  Example 6
The membrane forming process of Example 4 was repeated except that the
ratio of Resin R4308 to nylon 66 in the casting solution was adjusted
to 1.5 weight percent, i.e., 1.5 parts Resin R4308 to 100 parts nylon
66. Water was introduced into the in-line mixer at the rate of 28.7
grams per minute and the casting composition temperature was maintained
at 47.1°C. The web, moving at 18.3 metres per minute, was immersed into
a bath containing 50.1 weight percent formic acid and the balance
water. The membrane was then further processed as previously described
until ready for testing or filter application.
  Example 7
The membrane forming process of Example 4 was repeated except that the
ratio of Resin R4308 to nylon 66 was adjusted to 1.25 weight percent in
the casting solution, i.e., 1.25 parts Resin R4308 to 100 parts nylon
66. Water was introduced into the mixer at 28.7 grams per minute and
the casting composition temperature was maintained at 48.0°C. The web,
moving at 16.5 metres per minute, was immersed into a bath containing
50.2 weight percent formic acid, balance water, and then further
processed as previously described until ready for testing or filter
application.
  Example 8
The method of Example 1 was repeated, but with the casting solution
made up of about 487 parts of formic acid, 58.9 parts of a 2 weight
percent activated solution of Resin R4308 in water. 9.9 parts of water
and 94.0 parts of nylon 66 resin. The casting solution was pumped at a
rate of 500 grams per minute into an in-line mixer operating at 2648
RPM while water was injected into it at a rate of 25.3 grams per
minute. The resulting casting composition was maintained at 53.7°C and
cast onto the polyester web moving at 10.4 metres per minute. The web
was then immersed into a bath containing 54 percent formic acid, the
balance water, and then further processed as previously described until
ready for filter application or testing.
  Example 9
The method of Example 8 was repeated except that the speed of the
in-line mixer was 2631 RPM and water was injected into the mixer at a
rate of 12.0 grams per minute. The resulting casting composition was
maintained at 55.9°C before casting onto the polyester web moving at
9.1 metres per minute. The web was then immersed into a bath containing
54 percent formic acid, the balance water, and then processed as
hereinbefore described until ready for filter application or testing.
  Example 10
The method of Example 8 was repeated except that the speed of the
in-line mixer was 2719 RPM and water was injected into the mixer at a
rate of 4.4 grams per minute. The resulting casting composition was
maintained at 58.7°C and cast onto the polyester web moving at 6.7
metres per minute. The web was immersed into a bath containing 54
percent formic acid, the balance water, and then further processed as
hereinbefore described until ready for filter application or testing.
  Example 11
The method of Example 8 was repeated except that the speed of the
in-line was 2651 RPM and water was injected into the mixer at a rate of
6.5 grams per minute. The resulting casting composition was maintained
at 57.4°C and cast onto the polyester web moving at 7.9 metres per
minute. The web was immersed into a bath containing 54 percent formic
acid, the balance water and then further processed as hereinbefore
described until ready for filter application or testing.
  Example 12
The method of Example 2 was repeated except that the speed of the
in-line was 2719 RPM and water was injected into the mixer at a rate of
7.5 grams per minute. The resulting casting composition was maintained
at a temperature of 57.3°C and cast onto the polyester web moving at
6.7 metres per minute. The web was immersed into a bath containing 55
percent formic acid, the balance water, and was then further processed
as described hereinbefore until ready for filter application or
testing.
The pore diameters of the membranes of Examples 1ـ12 and a Control
membrane, prepared from nylon 66 without added membrane surface
modifying polymer, were determined by KL measurement as described in
U.S. Patent Specification 4,340,479 with the results set out in Table 1
below. The zeta potentials and adsorption capacities of the membranes
were also determined, by tests (a) and (b) above respectively, with the
results set out in Table 1.
The data in Table 1 above demonstrate that the present process of
preparing surface modified, microporous, hydrophilic membranes produces
membranes with positive zeta potentials in alkaline pH. Furthermore,
the data in the table demonstrate that membranes with widely differing
physical pore diameters can be prepared by this process. Moreover, the
listed adsorption capacities for 0.038 µm latex spheres, particles
whose diameter is much smaller than the pore size (pore diameter) of
these membranes, demonstrates the greatly enhanced particulate removal
efficiencies of these membranes compared to the control membrane, a
microporous, hydrophilic nylon 66 membrane made by the process of U.S.
Patent Specification No. 4,340,479. Consequently, the membranes are
superior to unmodified membranes in ultra-fine filtration applications.
The membrane of Example 12 was also tested for its ability to remove
the bacterium Serratia Marcescens from aqueous suspension by the
previously described Bacterial Titer Reduction Test Method (test (c)
above). For comparative purposes, an identical membrane, but prepared
without the added membrane surface modifying polymer, was included in
the test evaluation and is designated as Control in Table II.
The results in the above table demonstrate the greatly enhanced (by
nearly 100,000 fold) bacterial removal efficiency of filter membranes
prepared by the addition of the modifying polymer when compared to a
similar Control Membrane but without surface modification.
The membrane of Example 3 was tested for its ability to remove E. Coli
endotoxin from aqueous suspension by the previously described Endotoxin
Titer Reduction Test Method (test (d)). This endotoxin is believed to
be of molecular dimensions and exists in rod forms of about 0.001
micrometers in diameter. For comparative purposes, a similar membrane
prepared without the added membrane surface modifying polymer was
included in the test evaluations and is listed as Control in Table III.
Surprisingly, the membranes in accordance with the present invention
show extremely large improvements in the removal efficiency of
bacterial endotoxins when compared to unmodified membranes. The
presence of low levels of membrane surface modifying polymer produces
about a 100,000 fold increase in the endotoxin removal efficiency of
the membrane.
Unexpectedly, in addition to being able to remove unwanted materials of
biological activity, the membranes are able to decrease the adsorptive
removal of certain desirable components of filterable pharmaceutical
preparations. For example, membranes prepared by the method of Example
2 were tested for their ability to pass a solution of benzalkonium
chloride, a commonly used preservative in pharmaceutical products,
without undue reduction in the concentration of this substance. An
aqueous solution of 0.004 percent by weight of benzalkonium chloride
was passed through two layers of 47 mm diameter discs at a rate of 0.7
litres per minute per 929 cm² and the concentration of the preservative
in the effluent relative to that of the influent was determined as a
function of throughput volume. For the purpose of comparison, a
commercial nylon 66 membrane of the same pore size rating, designated
here as Control, was similarly tested.
The data in Table IV above illustrates that the effluent of filter
membranes reaches acceptable levels substantially before the effluent
of the Control. This is of great benefit when filtering such
pharmaceutical preparations because there is less wastage of the
required amount of preservative.
 General method II
Preparation by batch procedure of the membranes of Examples 13 and 14
In the following examples, polyamide membranes were prepared containing
different membrane surface modifying polymers using the following batch
procedure. Membrane casting resin solutions were prepared by dissolving
nylon 66 resin pellets of the same nylon 66 as used in Examples 1ـ12
(or other polyamide as specified in the examples) in a solution of
formic acid and the desired surface modifying polymer. Dissolution took
place with stirring at about 500 RPM in a jacketed resin kettle
maintained at 30°C. When dissolution was complete (usually within 3
hours), a non-solvent, water, was added to the solution in an amount
sufficient to adjust the final concentration of materials to that given
in each example. The water was pumped in at a rate of about 2 ml/min
through an orifice about 1 mm in diameter located under the surface of
the solution at a point about 1 cm from the stirring blade. Stirring
was maintained at about 500 RPM during addition of the water to induce
nucleation.
The casting composition was filtered through a 10 micrometer filter,
after which about 40 grams of the resulting casting composition was
spread out onto a clean glass plate by means of an adjustable gap
doctor blade. The film was then promptly immersed in a bath containing
formic acid and water in the amounts given in the examples below.
The membranes were held immersed in the bath for several minutes and
were then stripped from the glass plate. The membranes were washed in
water to remove residual formic acid and were oven-dried for 15 minutes
at 96 degrees C. while restrained in a frame to prevent shrinkage. The
flat membrane sheets were then used for filter applications or for
testing.
  Example 13
A membrane was prepared according to General Method II with the surface
modifying polymer being Polycup (Registered Trade Mark) 1884, a
polyamido/polyamino-epichlorohydrin as described above having (1) a
specific gravity of 1.12 and (2) a viscosity of 3250 mPas (325
centipoise) as a 35% aqueous solution. The casting solution contained
about 74.2 weight percent formic acid, 10.0 percent water, 14.3 weight
percent nylon 66 and 1.43 weight percent Polycup, (Registered Trade
Mark) 1884. The casting composition was spread as a film 0.038 cm thick
on a glass plate and immersed in a bath containing 54% by weight formic
acid, the balance being water. The membrane was then further processed
as described above under General Method II.
The membrane prepared was instantly wetted upon contact with water
(less than 1 second) and had a pore size of about 1 micrometer, as
determined by KL measurement. The zeta potential of the membrane was
found to be +2.8 mV at a pH of 8.0.
  Example 14
A membrane was prepared according to General Method II with the
polyamide resin being poly(hexamethylene azeleamide), (nylon 6,9), and
the surface modifying polymer being activated Resin R4308. The casting
solution contained about 65.4 weight percent formic acid, 17.7 weight
percent water, 16.0 weight percent nylon 6,9 resin and 0.8 weight
percent R4308 resin. The casting composition was spread as a film 0.053
cms thick on a glass plate and was immersed in a bath containing 60
percent by weight formic acid, the balance being water. The membrane
was then further processed as described above under General Method II.
The membrane of Example 14 was completely wetted immediately upon
contact with water (less than 1 second) and had a pore size of 8
micrometers as determined by K[L] measurement. The membrane had a zeta
potential of +3 mV at a pH of 8.0. Thus, the membrane prepared by this
method was microporous, hydrophilic and exhibited a positive zeta
potential at alkaline pH.
The continuous membrane preparation procedure (General Method I
described above) was used to prepare a number of membranes, each with a
pore size of 0.1 micrometer and containing various levels of added
Resin R4308. The preparation of these membranes is described in
Examples 1, 4, 5, 6 and 7. The membranes were prepared under identical
conditions from casting solutions containing from 4 weight percent
added Resin R4308 to as little as 1 weight percent. A similar membrane,
but without added Resin R4308 was also prepared by the process of U.S.
Patent Specification No. 4,340,479 as a comparative example and is
designated as Control in the discussion below.
These membranes were tested for their zeta potential at pH=7.5. All of
the membranes prepared with added Resin R4308 (from 1.25 to 4.1 weight
percent) had a strongly positive zeta potential. The Control membrane,
prepared without added Resin R4308, had a strong negative zeta
potential under the same measurement conditions. Thus, even low levels
added Resin R4308 were found to produce membranes with strong positive
zeta potential and improved filtration efficiency toward negatively
charged particulates in aqueous suspension.
The membranes of Examples 1, 4, 5, 6 and 7 were also converted into
filter cartridges by methods known in the art. These filter cartridges
were then flushed with 0.2 molar ammonium hydroxide at 3.52 kg per
square cm pressure across the cartridges for a period of 6 minutes,
followed by flushing with 1.5 liters of deionized water. They were then
dried for 12 hours at 79.4°C. The kilter cartridges were then tested
for their ability to deliver, within a short time after onset of
filtration, high purity effluent water of extremely low ionic content,
a requirement for the filtration of electronics grade water. For
comparative purposes, a filter cartridge containing the Control
membrane, prepared under similar conditions but without the added
surface modifying polymer, was included in the test evaluation and is
designed Control in Table V. The times for the effluent of these filter
cartridges to reach a resistivity of 14 megaohms, as measured by the
Resistivity Test described, above, along with the zeta potentials and
particulates adsorption capacities for the filter membranes are also
listed in Table V.
The results in the table show that the membranes have novel properties
useful in electronics water filtration when compared to prior art
membranes. The present surface modified membranes have positive zeta
potentials in alkaline media, vastly improved removal efficiencies for
ultrafine particulates and the ability to deliver purified effluent of
extremely low ionic content in rapid fashion after the onset of
filtration.
The relationship between (1) the time interval, from the onset of
filtration, required to produce the required filtrate water resistivity
of 14 megaohms/cm (rinse up time as designated in the Resistivity Test
described above) and (2) the percent added Resin R4308 is shown in the
sole Figure; a plot of rinse up time versus percent added Resin R4308
for each of the above filter cartridges. The figure illustrates that
the rinse up time diminishes linearly with decreasing levels of added
Resin R4308. The data in Table V show that at about one to one and
one-half percent added Resins R4308 the resulting membrane has a strong
positive zeta potential and a rinse up time substantially identical to
that of an unmodified membrane. This behaviour is highly desirable
since this membrane delivers high resistivity water efficiently and yet
provides enhanced filtration efficiency through electrostatic effects.
  Example 15
A surface modified, microporous, hydrophilic polyamide membrane
prepared according to the process of the subject invention from the
nylon 66 as used in Examples 1ـ12 and Resin R4308 and having a pore
rating of 2 micrometers was tested for its ability to remove haze and
haze precursors from commercial cherry brandy comprised of 40 percent
alcohol by volume. Prior to filtration, the brandy was chilled to about
0°C at which temperature it had a distinct, turbid appearance,
indicating the presence of an insoluble, dispersed phase of finely
divided haze. The test was carried out by passing the chilled cherry
brandy through a filter media comprised of two layers of the
microporous membrane described above. That is, a disc, 47 millimeters
in diameter, and comprised of two layers of the membrane described
above was mounted in a membrane holding device and the chilled cherry
brandy was then passed through this filter medium at a rate of 0.5
liters minute per 929 cm² (per square foot) of membrane surface area.
The initial pressure drop across the filter medium was 0.31 x 10^5 Pa
(4.5 psi). After 5 hours onstream, the pressure drop had increased to
0.5 0.496x10^5 Pa (7.2 psi). Over the 5 hours of filtration, the filter
effluent had a crystal clear appearance, without evidence of any haze.
The total volume of cherry brandy filtered over the 5 hour period
corresponded to 125 liters per 929 cm² (per square foot) of filter
medium. After filtration, the cherry brandy was allowed to warm up to
ambient temperature. No haze developed. Further, even after recooling,
the cherry brandy remained haze free.
This Example demonstrates that a membrane of the subject invention is
useful for treating alcoholic beverages to render them haze free and
stable against haze formation.
  Example 16
To further demonstrate the ability of the membranes in accordance with
the subject invention to operate in a clean manner as required in
certain filtration applications, such as the manufacture of near
theoretical resistivity water for electronics manufacture, a series of
elements AـD, as described below, were tested for "extractables" by the
process described below.
In this series of tests, corrugated filter elements of conventional
design having an effective surface area of about 0.46 m² (5 square
feet) were prepared by conventional means from three different
microporous membranes. These elements, labelled A through D in Table VI
below, were prepared from membranes which themselves were initially
prepared by the processes indicated below:
Element
A
 A hydrophilic, microporous polyamide membrane with a pore rating
 of 0.2 micrometers was prepared by the general process described
 in U.S. Patent Specification 4,340,479 from the same nylon 66 as
 used in Examples 1ـ12. The formed membrane was coated with Resin
 R4308 by impregnating the membrane with a 3 percent by weight
 solution of R4308 in water, the Resin R4308 having been
 activated according to the manufacturer's recommendations,
 following which the membrane was wiped to remove excess resin
 and thereafter formed into the filter element denoted as element
 A in Table VI below.
B
 The membrane of element B was prepared by the same process as
 described above with regard to the membrane of element A. This
 membrane also had a pore rating of 0.2 micrometers.
C
 This membrane was prepared by the cocasting process of the
 subject invention from (1) the same nylon 66 as the membranes of
 elements A and B above and (2) Resin R4308. The resulting
 membrane also had a pore rating of 0.2 micrometers and was
 comprised of 98 percent polyamide and 2 percent by weight Resin
 R4308.
D (Control)
 This Control membrane was prepared by the process described in
 U.S. Patent 4,340,479 from the same nylon 66 as elements A and B
 above. The resulting membrane also had a pore rating of 0.2
 micrometers. It did not contain a modifying polymer as either
 (1) an integral part of the structure (as did element C) or (2)
 a coating component of the membrane (as in the case of elements
 A and B).
The filter elements A through D as described above were tested as
follows:
Elements A and B were each (separately) subjected to a leaching step by
passing 1.8962 litres (0.5 U.S. gallons) per minute per element of room
temperature, deionized water through the respective element for the
time specified in Table VI below. This leaching step was carried out in
an effort to remove as much soluble material from elements A and B as
possible. Neither element C nor element D was given the benefit of this
treatment.
After the water leaching step carried out with elements A and B, each
of the elements was individually subjected to a room temperature,
deionized water flush at the pressure and for the time specified in
Table VI below. Note that the flushing times and pressures resulted in
a total flow of water through each individual element of about 189
litres (50 U.S. gallons) (elements A and B) and 227 litres (60 U.S.
gallons) (elements C and D).
After the deionized water flushing step, the elements were dried at
96°C (205°F) for about 12 hours, autoclaved with steam at 121°C for
about 1 hour and then extracted (again, each element separately) with
deionized water. The extraction step was carried out by plugging the
bottom of each element and then placing each element in its own bath of
one and one-half litres of deionized water, following which each of the
elements was reciprocated in an up and down manner (with the top of the
filter element rising about 5 centimeters above the upper level of the
bath on the up-stroke) for 4 hours.
In each case, the water in the bath was then evaporated and the
non-volatile residue remaiming behind weighed to determine the
extractable material in each filter element. The values are set out in
Table VI below.
As can be seen from Table VI, elements C and D had substantially
reduced extractables compared with the elements prepared from coated
polyamide membranes (A and B). This was the case even though elements A
and B were given a deionized water leach (of 30 and 60 minutes
respectively). As can also be seen from Table VI, element C, prepared
by the cocast process in accordance with this invention, had a low
level of extractables comparable to the Control element D. However,
element C combines the desirable positive zeta potential at pH 7 (as
well as at higher pH levels) with the low extractables of the Control D
which has, for many purposes, the undesirable negative zeta potential
at pH 7 (and at higher pH values as well).
 Industrial applicability
The surface modified membranes hereinbefore described have been
demonstrated to be superior in many important filtration related
properties to untreated prior art membranes. They are also superior in
many respects to coated membranes, e.g., in improved efficiency in
utilization of the surface modifying polymer and in certain surface
properties of the comparative end products. They can be used for
filtering applications in their manufactured form, with or without the
incorporation of the substrate upon which they are found. Two or more
membranes can be combined or secured to each other to form multiple
layer membrane filter sheets or they may be converted into filter
elements by known methods and employed in filter cartridges, e.g., as
filter elements in the form of a corrugated sheet supported within a
conventional cartridge.
The membranes display positive zeta potentials over a broad pH range of
from about 3 to about 10 and show greatly enhanced removal efficiencies
toward negatively charged particles in aqueous suspension. Furthermore,
they have enhanced efficiency to remove bacteria and endotoxins from
aqueous fluids. Moreover, the improved physical and chemical properties
coupled with their ability to quickly deliver high purity effluent
water, free from microparticulate and ionic contaminants, makes them
particularly desirable for use in microelectronics manufacture.
These membranes find use in industry and the medical field for
treatment of water supplies for critical applications such as water for
injection into humans, in microelectronics manufacture for the reasons
discussed above, for the filtration of blood serum to help achieve
sterility, for filtration of parental fluids, and generally for any use
where an ion containing fluid must be filtered to a high degree of
clarity.